Galvanic isolation between electrical systems is an increasingly needed feature that has become of more importance in modern industrial equipment along with the proliferation of inexpensive, low voltage microprocessor systems used to control higher voltage systems. There are various existing elements that can isolate the systems electrically but that still allow coupling. Those elements may include a transformer to couple circuits magnetically, an RF signal to couple through a radiated energy, an opto-isolator using light energy or a capacitor using an electric field to couple circuits having differing operating voltages.
Although opto-isolators are a good solution for low speed communication, they are inefficient in DC to DC power transfer cases. Transformers are very efficient for use in DC to DC applications, but traditional transformers are expensive because of the winding structures and the amount of board space and package height consumed. Printed transformers, such as those printed on a PCB (Printed Circuit Board) exhibit some improvements in cost and repeatability over a wound transformer, however, these transformers use a large footprint on the circuit board and can also suffer degradation due to moisture or other contaminant absorption over time, resulting in higher leakages and lowered break down isolation capability. Additionally, it is desirable to design a transformer integrated into a package compatible form, so that it could be placed by traditional package assembly equipment and would have an additional layer of protection from external contaminants. In addition it is desired to arrange the integrated transformer in a manner to boost efficiency and minimize losses while operating along with co-integrated circuits in an integrated package.
U.S. Patent Application Publication No. 2015/0069572, entitled “Multilayer High Voltage Isolation Barrier in an Integrated Circuit,” to Khanolkar et. al., published Mar. 12, 2015, which is co-owned with the present application and which is hereby incorporated by reference in its entirety herein, describes creating a galvanically isolated integrated circuit. In the above referenced published patent application, an integrated circuit transformer is disclosed that can be fabricated and assembled into an integrated circuit package to help alleviate some of the constraints and limitations of the traditional wound transformers, as well as addressing some of the limitations of prior known printed PCB transformers.
FIG. 1 depicts in a circuit schematic 100A and block diagram 100B a simple power supply with an isolation transformer. The simple example power supply of FIG. 1 has an input voltage Vin numbered 102 input to an oscillation circuit 120, an isolation transformer 110 and an output rectification circuit 130 producing an output voltage Vout numbered 104. The output circuit, called a tank circuit, also has an inductor 112. The power supply of FIG. 1 is a typical DC-DC converter circuit where the input circuit 120 oscillates with the input voltage Vin so that voltage can be transmitted through isolation transformer 110. The output voltage Vout, coupled through the transformer 110, is rectified by the output circuit 130. A block diagram 100B of this power supply is shown with Vin 102 feeding into the primary side oscillator circuit 120 which is coupled to an isolation transformer 110. The secondary side of the transformer winding is coupled to the rectification circuit 130 which produces the Vout 104. The power supply circuit of FIG. 1 features galvanic isolation via the transformer 110 so that the voltage and current domains on each side of the transformer are independent. In the above referenced published patent application US 2015/0069572, this circuit example is disclosed as being miniaturized into an integrated circuit package. The miniaturization is achieved primarily by creating a multilayer laminate isolation core on which the transformer 110 and tank coil 112 are fabricated. Separate components comprising the input circuit 120 and the output circuit 130 were then mounted in the same integrated circuit package as the transformer laminate. An integrated power supply circuit containing the laminate transformer is much less expensive than a power supply fabricated using traditional wound transformers. The resulting package takes up less board space, and it can be placed with common pick and place assembly equipment; all those factors lead to a lower cost, smaller product, which is desirable.
FIG. 2 depicts in a top view an integrated two layer air-core planar spiral transformer (a transformer laminate) 200. The term “air-core” used here refers to the manner in which the windings are constructed. In this arrangement, the magnetic field path is not influenced by any magnetic material around the transformer. In 200, a prior art transformer laminate as disclosed in the published application US 2015/0069572 referenced above is shown having an air-core transformer 210 and a printed coil 212, both as corresponding to like elements as seen in the circuit diagram in FIG. 1 and numbered as isolation transformer 110 and coil 112.
A cross section of transformer laminate 200 is oriented horizontally as shown by line 3-3′ with an illustration in FIG. 3.
In FIG. 3, illustrating the cross section of laminate 200, the primary and secondary coils numbered 310, 312 are shown in a vertically stacked arrangement to allow for the most direct magnetic coupling. The coils 310, 312 can be formed, for example of copper conductive material, or other electrically conductive material. For high voltage isolation, the core 319 and prepreg layers 315, 317, 321, 323, of the laminate 200 can be made from high pressure bismaleimide triazine (BT) laminates, for example. BT is noted for its high voltage breakdown strength and is available as a common semiconductor and electronics industry material that is used in copper clad laminates, circuit boards, and prepregs. Prepreg is a common industry term for a reinforcing laminate layer which has been pre-impregnated with a resin, typically epoxy resin. The prepreg material used for layers 315, 317, 321 and 323 over each side of core 319 in FIG. 3 includes curing agents and is ready to use without additional resin. Curing of the prepreg is accomplished by a combination of heat and pressure as prescribed by the manufacturer.
FIG. 4 depicts in a top view a prior art package leadframe 450 for use with the prior known isolation laminate described above. In FIG. 4, the leadframe 450 has dashed lines that indicate the general mounting position 452 for the transformer laminate 200 depicted in FIG. 2. A circuit die attach pad area is provided for an input circuit at position 460 and a second die attach pad area is provided for an output circuit 462 which, in combination with the transformer laminate 200 and with encapsulation (not shown in FIG. 4), can be used to create a packaged, integrated galvanic isolation circuit. In the leadframe 450 of FIG. 4, a void 454 is shown in the die attach pad area 452 that is to be located under the transformer laminate. This void 454 is utilized to allow the magnetic flux lines of the transformer laminate to circulate more freely and efficiently. Although this void 454 improves the coupling of the coils, it also limits the amount of heat that can be transferred from a package formed using the leadframe 450. In addition, the input and output circuit locations, 460 and 462, may be subject to high levels of EMI (Electro-Magnetic Interference) from the magnetic flux of the transformer laminate when it is used on the leadframe 450.
FIG. 5 depicts in an orthographic projection showing features of a prior art integrated power supply integrated circuit 501 utilizing a prior known transformer laminate for providing isolation. In FIG. 5, a leadframe 550 similar to the leadframe 450 in FIG. 4 holds an input circuit 520, a transformer laminate 500 with an isolation transformer 510 and an output circuit 530. These components are used for forming a simple power supply with galvanic isolation as diagramed in FIG. 1. Coil 512 on laminate 500 serves as the tank inductor such as is shown as 112 in the circuit diagram in FIG. 1. With the addition of encapsulation (not shown), these components can form an integrated power supply in an IC package utilizing a laminated transformer 500. The integration achieves a packaged power supply, for example, having significant size and cost advantages over non-integrated solutions.
Continuing improvements are therefore needed for methods and apparatus for integrated devices including improved isolation between circuit components to improve efficiency, increase power handling capacity, improve thermal conduction, and to reduce EMI.